Optical film

ABSTRACT

An optical film including a transparent polymer film (b) and a birefringent layer (a) formed of a polymer and laminated above the transparent polymer film (b) is provided. The birefringent layer (a) and the transparent polymer film (b) satisfy the following formula (1), the birefringent layer (a) satisfies the following formulae (2) and (3), and the polymer forming the birefringent layer (a) has a weight-average molecular weight in the range between 10,000 and 400,000 inclusive.
 
Δ n ( a )&gt;Δ n ( b )×10  (1)
 
1&lt;( nx−nz )/( nx−ny )  (2)
 
0.0005≦Δ n ( a )≦0.5  (3)
 
     This optical film can prevent the occurrence of iridescence, the occurrence of cracks, and the occurrence of variation in retardation.

TECHNICAL FIELD

The present invention relates to an optical film suitable for opticalcompensation of a liquid crystal cell.

BACKGROUND ART

Conventionally, a retardation plate has been used in various liquidcrystal displays in order to achieve optical compensation. As such aretardation plate, an optically uniaxial film or an optically biaxialfilm has been used, for example. The optically uniaxial film can beproduced, for example, by forming a particular polyimide into a film.The optical uniaxiality of the film is derived from the nature of thepolyimide itself, and the obtained film exhibits negative uniaxialoptical characteristics (see H8(1996)-511812 A, for example). On theother hand, the optically biaxial film has an excellent opticalcompensation function. For example, when the optically biaxial film isarranged as an optical film between a liquid crystal cell and apolarizer of a liquid crystal display, it can enhance the displaycharacteristics of the liquid crystal display, for example, by wideningits viewing angle. On this account, in recent years, instead of theoptically uniaxial film, the optically biaxial film is used as aretardation plate more and more widely. The optically biaxial film canbe produced, for example, by various polymer film stretching methods(see H3(1991)-33719 A, for example) and biaxially stretching methods(see H3(1991)-24502 A, for example). Also, there has been known aretardation plate in which a uniaxially stretched polymer film having apositive optical anisotropy and a biaxially stretched polymer filmhaving a negative optical anisotropy and whose in-plane retardation issmall are used in combination (see H4(1992)-194820 A, for example).

Disclosure of Invention

Although such an optically biaxial film produces an effect that itprovides a liquid crystal display achieving excellent contrast over awide viewing angle when used in the liquid crystal display, there hasbeen a problem in that it causes iridescence. Also, some opticallybiaxial films formed of particular polymers have a problem in thatcracks and/or variation in retardation may occur in the films.

Therefore, it is an object of the present invention to provide anoptical film having an optical biaxiality, capable of preventing theoccurrence of iridescence, the occurrence of cracks, variation inretardation, a poor appearance, and the like when used in variousdisplays such as a liquid crystal display, and further enhancing thedisplay characteristics of the displays.

The present invention provides an optical film including: a transparentpolymer film (b); and a birefringent layer (a) formed of a polymer andlaminated above the transparent polymer film (b). The birefringent layer(a) and the transparent polymer film (b) satisfy a formula (1) below,the birefringent layer (a) satisfies formulae (2) and (3) below, and thepolymer forming the birefringent layer (a) has a weight-averagemolecular weight in a range between 10,000 and 400,000 inclusive.Δn(a)>Δn(b)×10  (1)1<(nx−nz)/(nx−ny)  (2)0.0005 23 Δn(a)≦0.5  (3)

In the above formulae, Δn(a) is a birefringence of the birefringentlayer (a) and is represented by [(nx+ny)/2]−nz; Δn(b) is a birefringenceof the transparent polymer film (b) and is represented by[(nx′+ny′)/2]−nz′; nx, ny, and nz represent refractive indices in anX-axis direction, a Y-axis direction, and a Z-axis direction in thebirefringent layer (a), respectively, with the X-axis direction being anaxial direction exhibiting a maximum refractive index within a plane ofthe birefringent layer (a), the Y-axis direction being an axialdirection perpendicular to the X-axis within the plane, and the Z-axisdirection being a thickness direction perpendicular to the X-axis andthe Y-axis; and nx′, ny′, and nz′ represent refractive indices in anX-axis direction, a Y-axis direction, and a Z-axis direction in thetransparent polymer film (b), respectively, with the X-axis directionbeing an axial direction exhibiting a maximum refractive index within aplane of the transparent polymer film (b), the Y-axis direction being anaxial direction perpendicular to the X-axis within the plane, the Z-axisdirection being a thickness direction perpendicular to the X-axis andthe Y-axis.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of axial directions of a birefringent layer (a)in an optical film according to the present invention.

FIG. 2 is a schematic cross-sectional view showing an example of alaminated polarizing plate according to the present invention.

FIG. 3 is a schematic cross-sectional view showing another example of alaminated polarizing plate according to the present invention.

FIG. 4 is a schematic cross-sectional view showing an example of aliquid crystal display according to the present invention.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

The inventors of the present invention found out that, when an opticalfilm of the present invention including a transparent polymer film (b)and a birefringent layer (a) formed of a polymer and laminated above thetransparent polymer film (b) satisfies all the conditions represented byformulae (1) to (3), the occurrence of iridescence can be suppressed.The inventors of the present invention also found out that, when thepolymer forming the birefringent layer (a) has a weight-averagemolecular weight in a particular range, the occurrence of cracks,variation in retardation, and a poor appearance can be suppressed. Basedon these findings, the inventors arrived at the present invention.

In the present invention, it is necessary that Δn(a) as a birefringenceof the birefringent layer (a) and Δn(b) as a birefringence of thetransparent polymer film (b) satisfy the above formula (1).

Usually, the optical compensation is performed by the birefringent layer(a). Thus, in order to prevent the birefringence of the transparentpolymer film (b) from hindering the compensation in the optical design,the above relationship needs to be satisfied.

Preferably, Δn(a) and Δn(b) satisfy the relationship of Δn(a)>Δn(b)×15,because this allows an optical film that is still further visuallycompensated to be obtained. More preferably, Δn(a) and Δn(b) satisfy therelationship of Δn(a)>Δn(b)×20.

The axial directions of the refractive indices (nx, ny, nz) in thebirefringent layer (a) are indicated specifically by arrows in aschematic view of FIG. 1. As mentioned above, the refractive indices nx,ny, and nz indicate refractive indices in an X-axis direction, a Y-axisdirection, and a Z-axis direction, respectively. The X-axis direction isan axial direction exhibiting a maximum refractive index within theplane, the Y-axis direction is an axial direction perpendicular to the Xaxis within the plane, and the Z-axis direction is a thickness directionperpendicular to the X axis and the Y axis.

Furthermore, in the present invention, it is necessary that thebirefringent layer (a) satisfies the above formula (2). When the opticalfilm of the present invention satisfies 1<(nx−nz)/(nx−ny), thebirefringence in its thickness direction becomes greater than thosewithin a film plane. Thus, the optical film satisfying the aboverelationship is excellent in optical compensation of a liquid crystalcell, for example.

Furthermore, the value of (nx−nz)/(nx−ny) preferably satisfies(nx−nz)/(nx−ny)<100, because this allows, when the optical film of thepresent invention is used, for example, in a liquid crystal display, asufficient contrast ratio and more excellent viewing anglecharacteristics to be obtained. Still further, the value of(nx−nz)/(nx−ny) preferably satisfies 1<(nx−nz)/(nx−ny)≦80, more prefeaby1 (nx−nz) (nx−ny)≦50, to provide an optical film excellent in opticalcompensation. Furthermore, when the optical film is used in a verticallyaligned (VA) mode liquid crystal displays, it is preferable that thevalue of (nx−nz)/(nx−ny) satisfies 1≦(nx−nz)/(nx−ny)≦30.

In the present invention, it is necessary that the value of thebirefringence (Δn(a)) of the birefringent layer (a) satisfies the aboveformula (3), i.e., the value of the birefringence (Δn(a)) needs to be inthe range from 0.0005 to 0.5. When the value of the birefringence(Δn(a)) is 0.0005 or more, it is possible to obtain a thinnerbirefringent layer. On the other hand, when the value of thebirefringence (Δn(a)) is 0.5 or less, it is possible to control theretardation easily. In order to obtain an optical film with excellentproductivity, the value of the birefringence (Δn(a)) preferably rangesfrom 0.01 to 0.2, more preferably from 0.02 to 0.15.

In the present invention, the thickness of the birefringent layer (a) isnot particularly limited, but ranges, for example, from 0.1 to 50 μm,preferably from 0.5 to 30 μm, and more preferably from 1 to 20 μm, inorder to provide a uniform optical film with an excellentvisually-compensating function while reducing the thickness of a liquidcrystal display as much as possible.

The thickness of the transparent polymer film (b) can be determinedsuitably depending on the intended use and the like, but ranges, forexample, from 5 to 500 μm, preferably from 10 to 200 μm, and morepreferably from 15 to 150 μm, considering the strength of the opticalfilm, the reduction in the thickness of the optical film, and the like.

The birefringent layer (a) may be laminated on one or both surfaces ofthe transparent polymer film (b). The number of the birefringent layersmay be one or at least two for each surface. Moreover, the birefringentlayer (a) may be laminated directly on the transparent polymer film (b),or alternatively, an additional layer(s) may be arranged between thebirefringent layer (a) and the transparent polymer film (b).

The transparent polymer film (b) can be either a monolayer or a laminateincluding two or more layers. When the transparent polymer film is alaminate, layers included therein may be either the same or differentpolymer layers, depending on the intended use of the transparent polymerfilm (b), such as improving a strength, a heat resistance, or adhesionto the birefringent layer (a), for example.

The material for the birefringent layer (a) is not particularly limitedas long as the birefringent layer (a) obtained finally satisfies theabove-described respective conditions of the present invention. However,in order to obtain the birefringent layer (a) that satisfies thecondition (1) among these conditions, it is preferable to select thematerial for the birefringent layer (a), for example, depending on thematerial for the transparent polymer film (b) to be described later.Preferable example of the method of selecting the materials for thebirefringent layer (a) and the transparent polymer film (b) is asfollows: the material for the birefringent layer (a) is selected so thatthe birefringence of the birefringent layer (a) formed using theselected material becomes relatively high, whereas the material for thetransparent polymer film (b) is selected so that the birefringence ofthe transparent polymer film (b) formed using the selected materialbecomes relatively low.

The polymer used for forming the birefringent layer (a) in the presentinvention has a weight-average molecular weight (Mw) in the rangebetween 10,000 and 400,000 inclusive, where the weight-average molecularweight (Mw) is measured by gel permeation chromatography (GPC) using asa standard sample polyethylene oxide dissolved in a dimethylformamidesolvent. By using a polymer having a weight-average molecular weight(Mw) of 10,000 or more, a birefringent layer exhibiting a highbirefringence can be obtained, and in addition, the occurrence of crackscan be prevented. Furthermore, by using a polymer having aweight-average molecular weight (Mw) of 400,000 or less, variation inretardation can be prevented. The reason for this is that, in the casewhere the birefringent layer (a) is formed by applying a polymersolution, the viscosity of a solution of a polymer having aweight-average molecular weight (Mw) of 400,000 or less is not too high,thereby allowing a base or the like to be coated with the polymersolution easily so that a uniform birefringent layer (a) can be formed.When preparing a polymer solution using a polymer having aweight-average molecular weight (Mw) of 400,000 or less, the amount ofthe solvent used may be small because the solubility of the polymer ishigh. As a result, the coating layer can be made thin so that coatingcan be performed with high precision. The weight-average molecularweight (Mw) of a polymer forming the birefringent layer (a) preferablyranges between 10,000 and 300,000 inclusive, more preferably between10,000 and 200,000 inclusive.

The material for the birefringent layer (a) preferably is a non-liquidcrystalline polymer. Unlike a liquid crystal polymer, a non-liquidcrystalline polymer forms a film exhibiting an optical uniaxialitysatisfying nx>nz and also ny>nz according to its own nature, regardlessof the aligning property of a base. Thus, the base used is not limitedto an alignment base. For example, even when an unaligned base is used,the process for coating a surface of the base with an alignment film,laminating an alignment film on a surface of the base, or the like maybe omitted.

In the present invention, as a non-liquid crystalline polymer, at leastone polymer selected from the group consisting of polyamide, polyimide,polyester, polyetherketone, polyaryletherketone, polyamide imide, andpolyesterimide preferably is used. These polymers are suitable as amaterial for a biaxial film because of their excellent heat resistance,chemical resistance, hardness, and transparency. There is no particularlimitation on the non-liquid crystalline polymer to be used, and anyconventionally known polymer materials can be used suitably as long asthe characteristics of the optical film of the present invention aresatisfied. These polymers may be use either alone or in arbitrarycombination.

As the polyimide, it is preferable to use a polyimide that has a highin-plane aligning property and is soluble in an organic solvent.Specifically, examples of such a polyimide include a condensationpolymer product of 9,9-bis(aminoaryl)fluorene and an aromatictetracarboxylic dianhydride disclosed in JP 2000-511296 A, i.e., apolymer containing at least one repeating unit represented by theformula (1) below.

In the above formula (1), R³ to R⁶ are at least one substituent selectedindependently from the group consisting of hydrogen, halogen, a phenylgroup, a phenyl group substituted with 1 to 4 halogen atoms or a C₁₋₁₀alkyl group, and a C₁₋₁₀ alkyl group. Preferably, R³ to R⁶ are at leastone substituent selected independently from the group consisting ofhalogen, a phenyl group, a phenyl group substituted with 1 to 4 halogenatoms or a C₁₋₁₀ alkyl group, and a C₁₋₁₀ alkyl group.

In the above formula (1), Z is, for example, a C₆₋₂₀ quadrivalentaromatic group, and preferably is a pyromellitic group, a polycyclicaromatic group, a derivative of a polycyclic aromatic group, or a grouprepresented by the formula (2) below.

In the formula (2) above, Z′ is, for example, a covalent bond, a C(R⁷)₂group, a CO group, an O atom, an S atom, an SO₂ group, an Si(C₂H₅)₂group, or an NR⁸ group. When there are plural Z's, they may be the sameor different. Also, w is an integer from 1 to 10. R⁷s independently arehydrogen or C(R⁹)₃. R⁸ is hydrogen, an alkyl group having from 1 toabout 20 carbon atoms, or a C₆₋₂₀ aryl group, and when there are pluralR⁸s, they may be the same or different. R⁹s independently are hydrogen,fluorine, or chlorine.

The above-mentioned polycyclic aromatic group may be, for example, aquadrivalent group derived from naphthalene, fluorene, benzofluorene, oranthracene. Further, a substituted derivative of the above-mentionedpolycyclic aromatic group may be the above-mentioned polycyclic aromaticgroup substituted with at least one group selected from the groupconsisting of, for example, a C₁₋₁₀ alkyl group, a fluorinatedderivative thereof, and halogen such as F and Cl.

Other than the above, homopolymer whose repeating unit is represented bythe general formula (3) or (4) below or polyimide whose repeating unitis represented by the general formula (5) below disclosed in JP8(1996)-511812 A may be used, for example. The polyimide represented bythe formula (5) below is a preferable mode of the homopolymerrepresented by the formula (3).

In the above general formulae (3) to (5), G and G′ each are a groupselected independently from the group consisting of, for example, acovalent bond, a CH₂ group, a C(CH₃)₂ group, a C(CF₃)₂ group, a C(CX₃)₂group (wherein X is halogen), a CO group, an O atom, an S atom, an SO₂group, an Si(CH₂CH₃)₂ group, and an N(CH₃) group, and G and G′ may bethe same or different.

In the above formulae (3) and (5), L is a substituent, and d and eindicate the number of substitutions therein. L is, for example,halogen, a C₁₋₃ alkyl group, a halogenated C₁₋₃ alkyl group, a phenylgroup, or a substituted phenyl group, and when there are plural Ls, theymay be the same or different. The above-mentioned substituted phenylgroup may be, for example, a substituted phenyl group having at leastone substituent selected from the group consisting of halogen, a C₁₋₃alkyl group and a halogenated C₁₋₃ alkyl group. Also, theabove-mentioned halogen may be, for example, fluorine, chlorine,bromine, or iodine. d is an integer from 0 to 2, and e is an integerfrom 0 to 3.

In the above formulae (3) to (5), Q is a substituent, and f indicatesthe number of substitutions therein. Q may be, for example, an atom or agroup selected from the group consisting of hydrogen, halogen, an alkylgroup, a substituted alkyl group, a nitro group, a cyano group, athioalkyl group, an alkoxy group, an aryl group, a substituted arylgroup, an alkyl ester group and a substituted alkyl ester group and,when there are plural Qs, they may be the same or different. Theabove-mentioned halogen may be, for example, fluorine, chlorine,bromine, or iodine. The above-mentioned substituted alkyl group may be,for example, a halogenated alkyl group. Also, the above-mentionedsubstituted aryl group may be, for example, a halogenated aryl group. fis an integer from 0 to 4, and g and h respectively are an integer from0 to 3 and an integer from 1 to 3. Furthermore, it is preferable that gand h are larger than 1.

In the above formula (4), R¹⁰ and R¹¹ are groups selected independentlyfrom the group consisting of hydrogen, halogen, a phenyl group, asubstituted phenyl group, an alkyl group, and a substituted alkyl group.It is particularly preferable that R¹⁰ and R¹¹ independently are ahalogenated alkyl group.

In the above formula (5), M¹ and M² may be the same or different and,for example, halogen, a C₁₋₃ alkyl group, a halogenated C₁₋₃ alkylgroup, a phenyl group, or a substituted phenyl group. Theabove-mentioned halogen may be, for example, fluorine, chlorine,bromine, or iodine. The above-mentioned substituted phenyl group may be,for example, a substituted phenyl group having at least one substituentselected from the group consisting of halogen, a C₁₋₃ alkyl group, and ahalogenated C₁₋₃ alkyl group.

A specific example of polyimide represented by the formula (3) includespolyimide represented by the formula (6) below.

Moreover, the above-mentioned polyimide may be, for example, copolymerobtained by copolymerizing acid dianhydride and diamine other than theabove-noted skeleton (the repeating unit) suitably.

The above-mentioned acid dianhydride may be, for example, aromatictetracarboxylic dianhydride. The aromatic tetracarboxylic dianhydridemay be, for example, pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, naphthalene tetracarboxylic dianhydride,heterocyclic aromatic tetracarboxylic dianhydride, or 2,2′-substitutedbiphenyl tetracarboxylic dianhydride.

The pyromellitic dianhydride may be, for example, pyromelliticdianhydride, 3,6-diphenyl pyromellitic dianhydride,3,6-bis(trifluoromethyl)pyromellitic dianhydride,3,6-dibromopyromellitic dianhydride, or 3,6-dichloropyromelliticdianhydride. The benzophenone tetracarboxylic dianhydride may be, forexample, 3,3′,4,4′-benzophenone tetracarboxylic dianhydride,2,3,3′,4′-benzophenone tetracarboxylic dianhydride, or2,2′,3,3′-benzophenone tetracarboxylic dianhydride. The naphthalenetetracarboxylic dianhydride may be, for example,2,3,6,7-naphthalene-tetracarboxylic dianhydride,1,2,5,6-naphthalene-tetracarboxylic dianhydride, or2,6-dichloro-naphthalene-1,4,5,8-tetracarboxylic dianhydride. Theheterocyclic aromatic tetracarboxylic dianhydride may be, for example,thiophene-2,3,4,5-tetracarboxylic dianhydride,pyrazine-2,3,5,6-tetracarboxylic dianhydride, orpyridine-2,3,5,6-tetracarboxylic dianhydride. The 2,2′-substitutedbiphenyl tetracarboxylic dianhydride may be, for example,2,2′-dibromo-4,4′,5,5′-biphenyl tetracarboxylic dianhydride,2,2′-dichloro-4,4′,5,5′-biphenyl tetracarboxylic dianhydride, or2,2′-bis(trifluoromethyl)-4,4′,5,5′-biphenyl tetracarboxylicdianhydride.

Other examples of the aromatic tetracarboxylic dianhydride may include3,3′,4,4′-biphenyl tetracarboxylic dianhydride,bis(2,3-dicarboxyphenyl)methane dianhydride,bis(2,5,6-trifluoro-3,4-dicarboxyphenyl)methane dianhydride,2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride,4,4′-bis(3,4-dicarboxyphenyl)-2,2-diphenylpropane dianhydride,bis(3,4-dicarboxyphenyl)ether dianhydride, 4,4′-oxydiphthalicdianhydride, bis(3,4-dicarboxyphenyl)sulfonic dianhydride,3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride,4,4′-[4,4′-isopropylidene-di(p-phenyleneoxy)]bis(phthalic dianhydride),N,N-(3,4-dicarboxyphenyl)-N-methylamine dianhydride, andbis(3,4-dicarboxyphenyl)diethylsilane dianhydride.

Among the above, the aromatic tetracarboxylic dianhydride preferably is2,2′-substituted biphenyl tetracarboxylic dianhydride, more preferablyis 2,2′-bis(trihalomethyl)-4,4′,5,5′-biphenyl tetracarboxylicdianhydride, and further preferably is2,2′-bis(trifluoromethyl)-4,4′,5,5′-biphenyl tetracarboxylicdianhydride.

The above-mentioned diamine may be, for example, aromatic diamine.Specific examples thereof include benzenediamine, diaminobenzophenone,naphthalenediamine, heterocyclic aromatic diamine, and other aromaticdiamines.

The benzenediamine may be, for example, diamine selected from the groupconsisting of benzenediamines such as o-, m-, and p-phenylenediamine,2,4-diaminotoluene, 1,4-diamino-2-methoxybenzene,1,4-diamino-2-phenylbenzene, and 1,3-diamino-4-chlorobenzene. Examplesof the diaminobenzophenone may include 2,2′-diaminobenzophenone and3,3′-diaminobenzophenone. The naphthalenediamine may be, for example,1,8-diaminonaphthalene or 1,5-diaminonaphthalene. Examples of theheterocyclic aromatic diamine may include 2,6-diaminopyridine,2,4-diaminopyridine, and 2,4-diamino-S-triazine.

Further, other than the above, the aromatic diamine may be4,4′-diaminobiphenyl, 4,4′-diaminodiphenyl methane,4,4′-(9-fluorenylidene)-dianiline,2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl,3,3′-dichloro-4,4′-diaminodiphenyl methane,2,2′-dichloro-4,4′-diaminobiphenyl, 2,2′,5,5′-tetrachlorobenzidine,2,2-bis(4-aminophenoxyphenyl)propane, 2,2-bis(4-aminophenyl)propane,2,2-bis(4-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 4,4′-diaminodiphenyl ether, 3,4′-diamino diphenyl ether,1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene,1,4-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)biphenyl,4,4′-bis(3-aminophenoxy)biphenyl, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3,-hexafluoropropane, 4,4′-diaminodiphenyl thioether, or 4,4′-diaminodiphenylsulfone.

The polyetherketone may be, for example, polyaryletherketone representedby the general formula (7) below, which is disclosed in JP 2001-49110 A.

In the above formula (7), X is a substituent, and q is the number ofsubstitutions therein. Also, a indicates the number of substitutions ina fluorine atom. X is, for example, a halogen atom, a lower alkyl group,a halogenated alkyl group, a lower alkoxy group, or a halogenated alkoxygroup, and when there are plural Xs, they may be the same or different.

The halogen atom may be, for example, a fluorine atom, a bromine atom, achlorine atom, or an iodine atom, and among these, a fluorine atom ispreferable. The lower alkyl group preferably is a C₁₋₆ lower straightchain alkyl group or a C₁₋₆ lower branched chain alkyl group and morepreferably is a C₁₋₄ straight or branched chain alkyl group, forexample. More specifically, it preferably is a methyl group, an ethylgroup, a propyl group, an isopropyl group, a butyl group, an isobutylgroup, a sec-butyl group, or a tert-butyl group, and particularlypreferably is a methyl group or an ethyl group. The halogenated alkylgroup may be, for example, a halide of the above-mentioned lower alkylgroup such as a trifluoromethyl group. The lower alkoxy group preferablyis a C₁₋₆ straight or branched chain alkoxy group and more preferably isa C₁₋₄ straight or branched chain alkoxy group, for example. Morespecifically, it further preferably is a methoxy group, an ethoxy group,a propoxy group, an isopropoxy group, a butoxy group, an isobutoxygroup, a sec-butoxy group, or a tert-butoxy group, and particularlypreferably is a methoxy group or an ethoxy group. The halogenated alkoxygroup may be, for example, a halide of the above-mentioned lower alkoxygroup such as a trifluoromethoxy group.

In the above formula (7), q and a is an integer from 0 to 4. In theformula (7), it is preferable that q=0 and a carbonyl group and anoxygen atom of an ether that are bonded to both ends of a benzene ringare present at para positions.

Also, in the above formula (7), R¹ is a group represented by the formula(8) below, and m is an integer of 0 or 1.

In the above formula (8), X′ is a substituent and is the same as X inthe formula (7), for example. In the formula (8), when there are pluralX's, they may be the same or different. q′ indicates the number ofsubstitutions in the X′ and is an integer from 0 to 4, preferably, q′=0.In addition, p is an integer of 0 or 1. b indicates the number ofsubstitutions in a fluorine atom and is an integer from 0 to 4.

In the formula (8), R² is a divalent aromatic group. This divalentaromatic group is, for example, an o-, m-, or p-phenylene group or adivalent group derived from naphthalene, biphenyl, anthracene, o-, m-,or p-terphenyl, phenanthrene, dibenzofuran, biphenyl ether, or biphenylsulfone. In these divalent aromatic groups, hydrogen that is bondeddirectly to the aromatic may be substituted with a halogen atom, a loweralkyl group, or a lower alkoxy group. Among them, the R² preferably isan aromatic group selected from the group consisting of the formulae (9)to (15) below.

In the above formula (7), the R¹ preferably is a group represented bythe formula (16) below, wherein R² and p are equivalent to those in theabove-noted formula (8).

Furthermore, in the formula (7), n indicates a degree of polymerizationranging, for example, from 2 to 5000 and preferably from 5 to 500. Thepolymerization may be composed of repeating units with the samestructure or those with different structures. In the latter case, thepolymerization form of the repeating units may be a block polymerizationor a random polymerization.

Moreover, it is preferable that an end on a p-tetrafluorobenzoylenegroup side of the polyaryletherketone represented by the formula (7) isfluorine and an end on an oxyalkylene group side thereof is a hydrogenatom. Such a polyaryletherketone can be represented by the generalformula (17) below, for example. In the formula below, n indicates adegree of polymerization as in the formula (7).

Specific examples of the polyaryletherketone represented by the formula(7) may include those represented by the formulae (18) to (21) below,wherein n indicates a degree of polymerization as in the formula (7).

Other than the above, the polyamide or polyester may be, for example,polyamide or polyester described by JP 10(1998)-508048 A, and theirrepeating units can be represented by the general formula (22) below.

In the above formula (22), Y is O or NH. E is, for example, at least onegroup selected from the group consisting of a covalent bond, a C₂alkylene group, a halogenated C₂ alkylene group, a CH₂ group, a C(CX₃)₂group (wherein X is halogen or hydrogen), a CO group, an O atom, an Satom, an SO₂ group, an Si(R)₂ group, and an N(R) group, and Es may bethe same or different. In the above-mentioned E, R is at least one of aC₁₋₃ alkyl group and a halogenated C₁₋₃ alkyl group and present at ameta position or a para position with respect to a carbonyl functionalgroup or a Y group.

Further, in the above formula (22), A and A′ are substituents, and t andz respectively indicate the numbers of substitutions therein.Additionally, p is an integer from 0 to 3, q is an integer from 1 to 3,and r is an integer from 0 to 3.

The above-mentioned A is selected from the group consisting of, forexample, hydrogen, halogen, a C₁₋₃ alkyl group, a halogenated C₁₋₃ alkylgroup, an alkoxy group represented by OR (wherein R is the group definedabove), an aryl group, a substituted aryl group by halogenation or thelike, a C₁₋₉ alkoxycarbonyl group, a C₁₋₉ alkylcarbonyloxy group, aC₁₋₁₂ aryloxycarbonyl group, a C₁₋₁₂ arylcarbonyloxy group and asubstituted derivative thereof, a C₁₋₁₂ arylcarbamoyl group, and a C₁₋₁₂arylcarbonylamino group and a substituted derivative thereof. When thereare plural As, they may be the same or different. The above-mentioned A′is selected from the group consisting of, for example, halogen, a C₁₋₃alkyl group, a halogenated C₁₋₃ alkyl group, a phenyl group, and asubstituted phenyl group and when there are plural A's, they may be thesame or different. A substituent on a phenyl ring of the substitutedphenyl group can be, for example, halogen, a C₁₋₃ alkyl group, ahalogenated C₁₋₃ alkyl group, or a combination thereof. The t is aninteger from 0 to 4, and the z is an integer from 0 to 3.

Among the repeating units of the polyamide or polyester represented bythe formula (22) above, the repeating unit represented by the generalformula (23) below is preferable.

In the formula (23), A, A′, and Y are those defined by the formula (22),and v is an integer from 0 to 3, preferably is an integer from 0 to 2.Although each of x and y is 0 or 1, not both of them are 0.

The transparent polymer film (b) of the present invention is notparticularly limited, and any conventionally known transparent films canbe used. For example, it is preferable that the transparent polymer film(b) is a protective film for a polarizer as will be described later,because this allows the optical film of the present invention to servealso as a protective film for a polarizing plate.

The material for the transparent polymer film (b) is not particularlylimited as long as the transparent polymer film (b) obtained finallysatisfies the above-described condition (1) of the present invention.However, as the material for the transparent polymer film (b), polymershaving an excellent transparency are preferable, and also thermoplasticresins are preferable because they are suitable for a treatment forstretching or shrinking the film as described later. Specific examplesof the thermoplastic resins include acetate resins such astriacetylcellulose (TAC), polyester resins, polyethersulfone resins,polysulfone resins, polycarbonate resins, polyamide resins, polyimideresins, polyolefin resins, acrylic resins, polynorbornene resins,cellulose resins, polyarylate resins, polystyrene resins, polyvinylalcohol resins, polyvinyl chloride resins, polyvinylidene chlorideresins, polyacrylic resins, and mixtures thereof. Liquid crystalpolymers and the like also can be used.

Furthermore, as a material for the transparent polymer film (b), amixture of a thermoplastic resin whose side chain has a substituted orunsubtituted imido group and a thermoplastic resin whose side chain hasa substituted or unsubtituted phenyl group and nitrile group, disclosedin JP 2001-343529 A (WO 01/37007), or the like also may be used.Specific examples of such a mixture include a resin compositioncontaining an alternating copolymer of isobutene and N-methylenemaleimide and an acrylonitrile-styrene copolymer. Among these materials,materials that can form transparent films whose birefringence isrelatively low are preferable, and more specifically, a mixture of athermoplastic resin whose side chain has a substituted or unsubtitutedimido group and a thermoplastic resin whose side chain has a substitutedor unsubtituted phenyl group and nitrile group is preferable.

The transparent polymer film (b) may be subjected a treatment forimparting an optical anisotropy to the film. For example, thetransparent polymer film (b) may be stretched previously. As the methodof stretching the transparent polymer film (b), tenter transversestretching and biaxial stretching in which the stretch ratio in the longaxis direction is lower than that in the short axis direction arepreferable. The biaxial stretching can be selected from simultaneousbiaxial stretching that uses a tenter alone, and sequential biaxialstretching that uses rollers and a tenter. Though the stretch ratiovaries depending on the stretching method, the polymer film may bestretched 1 to 200%, for example. The heating temperature whenstretching the polymer film is selected suitably depending on the glasstransition point (Tg) of the polymer film in use, the kinds of additivesin the polymer film, and the like, but may be, for example, in the rangefrom 80° C. to 250° C., preferably from 120° C. to 220° C., andparticularly preferably from 140° C. to 200° C. Especially, it ispreferable that the temperature for stretching the polymer film issubstantially equal to or higher than Tg of the polymer film.

Other than the above, a mixture containing a thermoplastic resin whoseside chain has a substituted or unsubtituted imido group and athermoplastic resin whose side chain has a substituted or unsubtitutedphenyl group and nitrile group, liquid crystal polymers, and the likealso may be used.

Moreover, it is preferable that the transparent polymer film (b) is apolarizer including a polyvinyl alcohol-based film as will be describedlater, because this allows the optical film of the present invention toserve also as a polarizing plate.

An optical film according to the present invention can be produced inthe following manner, for example. First, the predetermined polymer forforming a birefringent layer (a) is applied onto a transparent polymerfilm (b) to form a precursor layer of the birefringent layer (a). Thetransparent polymer film may have been subjected to a stretchingtreatment as described above. The method of applying the polymer is notparticularly limited, but may be a method of applying the polymer thathas been heated and melted, a method of applying a polymer solutionprepared by dissolving the polymer in a solvent, or the like, forexample. Among these, the method of applying a polymer solution ispreferable because of its excellent workability and its opticalanisotropy controllability.

The thickness of the birefringent layer (a) formed can be adjustedthrough the process for applying the polymer. For example, in the methodof applying a polymer solution, the thickness of the birefringent layer(a) can be adjusted by adjusting the amount of the polymer applied perarea (cm²) of the transparent polymer film (b).

The solvent of the polymer solution is not particularly limited as longas it can dissolve the polymer, and can be, for example, halogenatedhydrocarbons such as chloroform, dichloromethane, carbon tetrachloride,dichloroethane, tetrachloroethane, trichloroethylene,tetrachloroethylene, chlorobenzene, and orthodichlorobenzene; phenolssuch as phenol and parachlorophenol; aromatic hydrocarbons such asbenzene, toluene, xylene, methoxybenzene, and 1,2-dimethoxybenzene;ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone,cyclohexanone, cyclopentanone, 2-pyrrolidone, andN-methyl-2-pyrrolidone; esters such as ethyl acetate and butyl acetate;alcohols such as t-butyl alcohol, glycerin, ethylene glycol, triethyleneglycol, ethylene glycol monomethyl ether, diethylene glycol dimethylether, propylene glycol, dipropylene glycol, and2-methyl-2,4-pentanediol; amides such as dimethylformamide anddimethylacetamide; nitriles such as acetonitrile and butyronitrile;ethers such as diethyl ether, dibutyl ether, and tetrahydrofuran; orcarbon disulfide, ethyl cellosolve, or butyl cellosolve. It is possibleto use one of these solvents alone or a mixture of two or more solvents.

Preferably, the polymer solution has a viscosity allowing the solutionto be applied easily. The reason for this is that, if the polymersolution can be applied easily, it is possible to form a uniformbirefringent layer (a) as described above. The viscosity ranges, forexample, from 0.1 to 12 Pa·s, preferably from 1 to 10 Pa·s, and morepreferably from 1 to 5 Pa·s.

Although the concentration of the polymer in the polymer solution is notparticularly limited, it preferably is adjusted considering theweight-average molecular weight of the non-liquid crystalline polymerused so that the viscosity of the solution would be in theabove-described range. The concentration of the polymer ranges, forexample, from 5 to 50 parts by weight, preferably 10 to 40 parts byweight with respect to 100 parts by weight of the solvent.

In the polymer solution, various additives such as a stabilizer, aplasticizer, metal and the like further may be blended as necessary.

Moreover, the polymer solution may contain other resins as long as thealigning property of the polymer does not drop considerably. Such resinscan be, for example, resins for general purpose use, engineeringplastics, thermoplastic resins, and thermosetting resins.

The resins for general purpose use can be, for example, polyethylene(PE), polypropylene (PP), polystyrene (PS), polymethyl methacrylate(PMMA), an ABS resin, an AS resin, or the like. The engineering plasticscan be, for example, polyacetate (POM), polycarbonate (PC), polyamide(PA: nylon), polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), or the like. The thermoplastic resins can be, forexample, polyphenylene sulfide (PPS), polyethersulfone (PES), polyketone(PK), polyimide (PI), polycyclohexanedimethanol terephthalate (PCT),polyarylate (PAR), liquid crystal polymers (LCP), or the like. Thethermosetting resins can be, for example, epoxy resins, phenolic novolacresins, or the like.

When the above-described other resins are blended in the polymersolution as mentioned above, the blend amount ranges, for example, from0 wt % to 50 wt %, preferably from 0 wt % to 30 wt %, with respect tothe polymer.

The coating treatment can be carried out by a suitable method such asspin coating, roller coating, flow coating, printing, dip coating, filmflow-expanding, bar coating, or gravure printing. In the coating,polymer layers can be superimposed as required.

Furthermore, the polymer may be applied onto the transparent polymerfilm (b) while applying a stress in one direction to the transparentpolymer film (b), or alternatively, the polymer may be applied whileblowing an air or the like on the transparent polymer film (b) from onedirection.

Then, the precursor layer is hardened, thus forming an optical film inwhich a birefringent layer (a) is formed on the transparent polymer film(b). The precursor layer may be hardened by carrying out natural drying(air drying) or heat drying, for example, at 25° C. to 180° C.,preferably from 80° C. to 170° C., and more preferably 60° C. to 150°C., after the precursor layer is applied onto the transparent polymerfilm (b). The time period for drying or heating the precursor layer isdetermined depending on the temperature at which the drying or heatingis performed, whether or not the solvent is used for forming theprecursor layer, the kind of the solvent used, and the like, but may be,for instance, for 0.5 to 30 minutes, preferably for 1 to 20 minutes, andmore preferably for 1 to 15 minutes.

The method of producing an optical film according to the presentinvention may further include stretching the laminate of the polymerfilm and the birefringent layer.

The method of stretching the laminate is not particularly limited, andfixed-end stretching and conventionally known methods can be applied,for example. Among these, tenter transverse stretching and biaxialstretching in which the stretch ratio in the long axis direction islower than that in the short axis direction are preferable. The biaxialstretching can be selected from simultaneous biaxial stretching thatuses a tenter alone, and sequential biaxial stretching that uses rollersand a tenter. Though the stretch ratio varies depending on thestretching method, the laminate may be stretched 1 to 200%, for example.The heating temperature when stretching the laminate is selectedsuitably depending on the glass transition point (Tg) of the transparentpolymer film in use, the kinds of additives in the transparent polymerfilm, and the like, but may be, for example, in the range from 80° C. to250° C., preferably from 120° C. to 220° C., and particularly preferablyfrom 140° C. to 200° C. Especially, it is preferable that thetemperature for stretching the laminate is substantially equal to orhigher than Tg of the polymer film.

Furthermore, the above-described method of producing an optical filmaccording to the present invention can be changed as follows, forexample. First, an optical film is produced in the same manner as in theabove except that a base, for example, is used instead of thetransparent polymer film (b). Then, the base on which a birefringentlayer (a) is formed directly is adhered to a transparent polymer film(b) so that the birefringent layer (a) faces the transparent polymerfilm (b) and thereafter, only the base is peeled off. As describedabove, an optical film of the present invention can be obtained bytransferring the birefringent layer (a) onto the transparent polymerfilm (b) and then peeling off the base, thus forming a laminate of thebirefringent layer (a) and the transparent polymer film (b).

As the base, any suitable materials can be used without any particularlimitations. For example, the base may be a polymer film having a lowglass transition point (Tg), a polymer film having a high elasticmodulus, a base having a linear expansion equal to or greater than thatof a material to be applied thereto, a base having a high thermalconductivity, a base having a high aspect ratio, a base having a smallthickness, or the like. Furthermore, in order to impart flexibility tothe base, any of the following methods can be employed, for example:drying the base without fixing the base, thereby making all the side ofthe base shrinkable; fixing at least one side of the base, therebymaking the sides other than the fixed side(s) of the base shrinkable;utilizing a linear expansion of a metal belt; controlling the shrinkageof a film by fixing the film with a tenter when conveying the film;increasing the shrinkage ratio of the base by expanding the basepreviously and then drying it; stretching the base before drying it,thereby causing shrinkage of the base due to hardening; and stretchingthe base during or after the process for drying it. However, it shouldbe noted that the method of imparting flexibility to the base is notlimited to these examples.

The thickness of the base can be determined suitably depending on theintended use and the like, but ranges, for example, from 5 to 500 μm,preferably from 10 to 200 μm, and more preferably from 15 to 150 μm,considering the strength of the optical film, the reduction in thethickness of the optical film, and the like.

The optical film according to the present invention may be used aloneor, if required, in combination with an additional birefringence film orthe like to form a laminate for various optical uses, e.g., opticalcompensating members of various liquid crystal display elements. Forexample, the optical film of the present invention may be used incombination with iodine-based or dyestuff-based polarizing plates (orpolarizers) that are produced industrially, so as to provide a laminatedpolarizing plate having a function of compensating and adjusting thebirefringence of a liquid crystal display element.

The polarizing plate that may optionally be used in combination with theoptical film according to the present invention is not particularlylimited. However, the polarizing plate basically is configured bylaminating a protective layer (film) on at least one surface of apolarizer.

The polarizer (polarizing film) is not particularly limited, but can bea film prepared by a conventionally known method of, for example, dyeingby allowing a film of various kinds to adsorb a dichroic material suchas iodine or a dichroic dye, followed by crosslinking, stretching, anddrying. Especially, films that transmit linearly polarized light whennatural light is made to enter those films are preferable, and filmshaving excellent light transmittance and polarization degree arepreferable. Examples of the film of various kinds in which the dichroicmaterial is to be adsorbed include hydrophilic polymer films such aspolyvinyl alcohol (PVA)-based films, partially-formalized PVA-basedfilms, partially-saponified films based on ethylene-vinyl acetatecopolymer, and cellulose-based films. Other than the above, polyenealigned films such as dehydrated PVA and dehydrochlorinated polyvinylchloride can be used, for example. Among them, a PVA-based film preparedby adsorbing iodine or a dichroic dye and aligning the film is usedpreferably. The thickness of the polarizing film generally is in therange from 1 to 80 μm, though it is not limited to this.

The protective layer (film) is not particularly limited, but can be aconventionally known transparent film. For example, transparentprotective films having excellent transparency, mechanical strength,thermal stability, moisture shielding property, and isotropism arepreferable. Specific examples of materials for such a transparentprotective layer include cellulose-based resins such astriacetylcellulose; transparent resins based on polyester,polycarbonate, polyamide, polyimide, polyethersulfone, polysulfone,polystyrene, polynorbornene, polyolefin, acrylic substances, acetate,and the like; mixtures of a thermoplastic resin whose side chain has asubstituted or unsubtituted imido group and a thermoplastic resin whoseside chain has a substituted or unsubtituted phenyl group and nitrilegroup; and liquid crystal polymers. Thermosetting resins orultraviolet-curing resins based on the acrylic substances, urethane,acrylic urethane, epoxy, silicones, and the like can be used as well.Among them, a TAC film having a surface saponified with alkali or thelike is preferable in light of the polarization property and durability.

Moreover, as the protective layer, the polymer film described in JP2001-343529 A (WO 01/37007) also can be used. The polymer material usedcan be a resin composition containing a thermoplastic resin whose sidechain has a substituted or unsubtituted imido group and a thermoplasticresin whose side chain has a substituted or unsubtituted phenyl groupand nitrile group, for example, a resin composition containing analternating copolymer of isobutene and N-methyl maleimide and anacrylonitrile-styrene copolymer. Alternatively, the polymer film may beformed by extruding the resin composition.

It is preferable that the protective layer is colorless, for example.More specifically, a retardation value (Rth) of the film in itsthickness direction as represented by the equation below preferablyranges from −90 nm to +75 nm, more preferably ranges from −80 nm to +60nm, and particularly preferably ranges from −70 nm to +45 nm. When theretardation value is within the range of −90 nm to +75 nm, coloration(optical coloration) of the polarizing plate, which is caused by theprotective film, can be solved sufficiently. In the equation below, nx,ny, and nz are the same as those described above, and d represents athickness of the protective film.Rz={[(nx+ny)/2]−nz}·d

The transparent protective layer further may have an opticallycompensating function. As such a transparent protective layer having theoptically compensating function, it is possible to use, for example, aknown layer used for preventing coloration caused by changes in avisible angle based on retardation in a liquid crystal cell or forwidening a preferable viewing angle. Specific examples include variousstretched films obtained by stretching the above-described transparentresins uniaxially or biaxially, an alignment film of a liquid crystalpolymer or the like, and a laminate obtained by providing an alignmentlayer of a liquid crystal polymer or the like on a transparent base.Among the above, the alignment film of a liquid crystal polymer ispreferable because a wide viewing angle with excellent visibility can beachieved. Particularly preferable is an optically compensatingretardation plate obtained by supporting an optically compensating layerwith the above-mentioned triacetylcellulose film or the like, where theoptically compensating layer is made of an incline-alignment layer of adiscotic or nematic liquid crystal polymer. This optically compensatingretardation plate can be a commercially available product, for example,“WV film” manufactured by Fuji Photo Film Co., Ltd. Alternatively, theoptically compensating retardation plate can be prepared by laminatingtwo or more layers of the retardation film and the film support oftriacetylcellulose film or the like so as to control the opticalcharacteristics such as retardation.

The thickness of the transparent protective layer is not particularlylimited and can be determined suitably according to retardation or aprotective strength, for example. The thickness of the transparentprotective layer is, for example, not more than 500 μm, preferably inthe range from 5 μm to 300 μm, and more preferably in the range from 5μm to 150 μm.

The transparent protective layer can be formed suitably by aconventionally known method such as a method of coating a polarizingfilm with the above-mentioned various transparent resins or a method oflaminating the transparent resin film, the optically compensatingretardation plate, or the like on the polarizing film, or can be acommercially available product.

The transparent protective layer further may be subjected to, forexample, a hard coating treatment, an antireflection treatment,treatments for anti-sticking, diffusion and anti-glaring and the like.The hard coating treatment aims to prevent scratches on the surfaces ofthe polarizing plate, and is a treatment of, for example, providing ahardened coating film that is formed of a curable resin and hasexcellent hardness and smoothness on a surface of the transparentprotective layer. The curable resin can be, for example,ultraviolet-curing resins of silicone base, urethane base, acrylic, andepoxy base. The treatment can be carried out by a conventionally knownmethod. The anti-sticking treatment aims to prevent adjacent layers fromsticking to each other. The antireflection treatment aims to preventreflection of external light on the surface of the polarizing plate, andcan be carried out by forming a conventionally known antireflectionlayer or the like.

The anti-glare treatment aims to prevent reflection of external light onthe polarizing plate surface from hindering visibility of lighttransmitted through the polarizing plate. The anti-glare treatment canbe carried out, for example, by providing microscopic asperities on asurface of the transparent protective layer by a conventionally knownmethod. Such microscopic asperities can be provided, for example, byroughening the surface by sand-blasting or embossing, or by blendingtransparent fine particles in the above-described transparent resin whenforming the transparent protective layer.

The above-described transparent fine particles may be silica, alumina,titania, zirconia, stannic oxide, indium oxide, cadmium oxide, antimonyoxide, or the like. Other than the above, inorganic fine particleshaving an electrical conductivity, organic fine particles including, forexample, crosslinked or uncrosslinked polymer particles, or the like canbe used as well. The average particle diameter of the transparent fineparticles ranges, for example, from 0.5 to 20 μm, though there is noparticular limitation. A blend ratio of the transparent fine particlesranges, for example, from 2 to 70 parts by weight, preferably from 5 to50 parts by weight with respect to 100 parts by weight of theabove-described transparent resin, though there is no particularlimitation.

An anti-glare layer in which the transparent fine particles are blendedcan be used as the transparent protective layer itself or provided as acoating layer or the like applied onto the transparent protective layersurface. Furthermore, the anti-glare layer also can function as adiffusion layer to diffuse light transmitted through the polarizingplate in order to widen the viewing angle (i.e., visually-compensatingfunction).

The antireflection layer, the anti-sticking layer, the diffusion layer,the anti-glare layer, and the like as mentioned above can be laminatedon the polarizing plate, as a sheet of optical layers including theselayers, separately from the transparent protective layer.

The method of laminating the respective components (the optical film,the polarizer, the transparent protective film, etc.) is notparticularly limited but a conventionally known method can be applied.In general, pressure-sensitive adhesives, adhesives, and the like asdescribed above can be used, and the kinds thereof can be determinedsuitably depending on the materials or the like of the components.Examples of the adhesives include polymer adhesives based on acrylicsubstances, vinyl alcohol, silicone, polyester, polyurethane, polyester,or the like and rubber-based adhesives. The above-mentionedpressure-sensitive adhesives and adhesives do not peel off easily evenwhen being exposed to moisture or heat, for example, and have excellentlight transmittance and polarization degree. More specifically, PVAadhesives are preferable when the polarizer is formed of a PVA-basedfilm, in light of stability of adhering treatment. These adhesive andpressure-sensitive adhesive may be applied directly to surfaces of thepolarizing layer and the transparent protective layer, or a layer of atape or a sheet formed of the adhesive or pressure-sensitive adhesivemay be arranged on the surfaces thereof. Further, when these adhesiveand pressure-sensitive adhesive are prepared as an aqueous solution, forexample, other additives or a catalyst such as an acid catalyst may beblended as necessary.

In the case of applying the adhesive, other additives or a catalyst suchas an acid catalyst further may be blended in the aqueous solution ofthe adhesive. Though the thickness of the adhesive layer is notparticularly limited, for example, it is from 1 nm to 500 nm, preferablyfrom 10 nm to 300 nm, and more preferably from 20 nm to 100 nm. It ispossible to adopt a known method of using an adhesive etc. such as anacrylic polymer or a vinyl alcohol-based polymer without any particularlimitations.

When forming a laminated polarizing plate by laminating a polarizingplate and an optical film, they may be laminated using any suitableadhesion means such as an adhesive layer or a pressure-sensitiveadhesive layer. However, the lamination method is not limited thereto.For example, the lamination method may be as follows. First, using as atransparent polymer film (b) a polymer film formed of triacetylcelluloseor the like, which generally is used as a protective layer for apolarizing plate, an optical film is produced by laminating abirefringent layer (a) on this transparent polymer film (b). Then, aprotective film formed of triacetylcellulose or the like is adhered toone surface of a polarizer, and the optical film is adhered to the othersurface of the polarizer. If the optical film is arranged so that thebirefringent layer (a) of the optical film faces the polarizer, thetransparent polymer film (b) of the optical film can be used as aprotective film for protecting one surface of the polarizing plate.

FIG. 2 shows an example of a configuration of a laminated polarizingplate including an optical film of the present invention and apolarizer. As shown in FIG. 2, in this laminated polarizing plate, apolarizer (2) is arranged between an optical film (1) and a protectivefilm (3).

FIG. 3 shows an example of a configuration of a laminated polarizingplate including an optical film of the present invention, a polarizer,and two protective films. As shown in FIG. 3, protective films (3) arearranged on the respective surfaces of a polarizer (2), thus forming apolarizing plate (11), and an optical film of the present inventionfurther is arranged on one of the protective films (3).

The adhesive (pressure-sensitive adhesive) used for lamination is notparticularly limited, and any suitable adhesives such as transparentpressure-sensitive adhesives based on acrylic substances, silicone,polyester, polyurethane, polyether, and rubbers can be used. In order toprevent optical characteristics of the optical film and the like fromchanging, adhesives that do not require a process at high temperaturewhen hardening or drying them are preferable, and those do not require along period for a hardening or drying treatment are desirable. Also, itis preferable to use an adhesive that does not cause the optical film tobe peeled off under a heating or humidifying condition, for example.

As described above, the optical film of the present invention may beused in combination with various retardation plates, diffusion-controlfilms, brightness-enhancement films, and the like. Examples of theretardation plates include those obtained by uniaxially or biaxiallystretching a p those subjected to a treatment for causing Z-axisalignment, and those obtained by applying a liquid crystal polymer.Examples of the diffusion-control films include films that controlviewing angles by utilizing diffusion, scattering, and refraction andfilms that control glaring, scattered light, and the like that affectthe resolution by utilizing diffusion, scattering, and refraction.Examples of the brightness-enhancement films includebrightness-enhancement films utilizing the selective reflection propertyof a cholesteric liquid crystal and provided with a λ/4 plate andscattering films utilizing an anisotropic scatter depending on thepolarization direction. Also, the optical film may be used incombination with a wire grid polarizer.

The laminated polarizing plate of the present invention can be usedsuitably for forming various liquid crystal displays, for example. Whenusing the laminated polarizing plate in a liquid crystal display or thelike, one or more other optical layers such as a reflection plate, asemitransparent reflection plate, and a brightness-enhancement film canbe laminated additionally as required via an adhesive layer or apressure-sensitive adhesive layer.

First, an example of a reflective polarizing plate or a semitransparentreflective polarizing plate will be described. The reflective polarizingplate is prepared by laminating further a reflection plate on alaminated polarizing plate according to the present invention, and thesemitransparent reflective polarizing plate is prepared by laminating asemitransparent reflection plate on a laminated polarizing plateaccording to the present invention.

In general, the reflective polarizing plate is arranged on a backside ofa liquid crystal cell in order to make a liquid crystal display(reflective liquid crystal display) that reflects incident light from avisible side (display side). The reflective polarizing plate isadvantageous in that, for example, it allows the liquid crystal displayto be thinned further because the necessity of providing a light sourcesuch as a backlight can be eliminated.

The reflective polarizing plate can be formed in any known manner suchas forming a reflection plate of metal or the like on one surface of apolarizing plate having a certain elastic modulus. More specifically,one example thereof is a reflective polarizing plate formed by mattingone surface (surface to be exposed) of a transparent protective layer ofthe polarizing plate as required, and providing the surface with adeposited film or a metal foil formed of a reflective metal such asaluminum.

Another example is a reflective polarizing plate prepared by forming, ona transparent protective layer having a surface with microscopicasperities due to microparticles contained in various transparentresins, a reflection plate corresponding to the microscopic asperities.The reflection plate having a surface with microscopic asperitiesdiffuses incident light irregularly so that directivity and glare can beprevented and irregularity in color tones can be controlled. Thereflection plate can be formed by attaching the metal foil or the metaldeposited film directly on the surface with asperities of thetransparent protective layer by any conventionally known methodsincluding deposition and plating, such as vacuum deposition, ionplating, and sputtering.

As mentioned above, the reflection plate can be formed directly on atransparent protective layer of a polarizing plate. Alternatively, areflecting sheet or the like formed by providing a reflecting layer on aproper film such as the transparent protective film can be used as thereflection plate. Since a typical reflecting layer of a reflection plateis made of a metal, it is preferably used in a state that the reflectingsurface of the reflecting layer is coated with the film, a polarizingplate, or the like, in order to prevent a reduction of the reflectancedue to oxidation, and furthermore, to allow the initial reflectance tobe maintained for a long period and to avoid the necessity of forming atransparent protective layer separately.

On the other hand, the semitransparent polarizing plate is provided byreplacing the reflection plate in the above-mentioned reflectivepolarizing plate by a semitransparent reflection plate. Examples of asemitransparent polarizing plate include a half mirror that reflects andtransmits light at the reflecting layer.

In general, such a semitransparent polarizing plate is arranged on abackside of a liquid crystal cell. In a liquid crystal display includingthe semitransparent polarizing plate, incident light from the visibleside (display side) is reflected to display an image when a liquidcrystal display is used in a relatively bright atmosphere, while in arelatively dark atmosphere, an image is displayed by using a built-inlight source such as a backlight on the backside of the semitransparentpolarizing plate. In other words, the semitransparent polarizing platecan be used to form a liquid crystal display that can save energy for alight source such as a backlight under a bright atmosphere, while abuilt-in light source can be used under a relatively dark atmosphere.

The following description is about an example of an optical film, alaminated polarizing plate, or the like prepared by further laminating abrightness-enhancement film on the optical film, the laminatedpolarizing plate, or the like according to the present invention.

A suitable example of the brightness-enhancement film is notparticularly limited, but it can be selected from a multilayer thin filmof a dielectric or a laminate of multiple thin films with variedrefraction aeolotropy that transmits linearly polarized light having apredetermined polarization axis while reflecting other light. Examplesof such a brightness-enhancement film include “D-BEF (trade name)”manufactured by 3M Co. Also, a cholesteric liquid crystal layer, morespecifically, an alignment film of a cholesteric liquid crystal polymeror an alignment liquid crystal layer fixed onto a supportive film basecan be used as a brightness-enhancement film. Such abrightness-enhancement film reflects either clockwise orcounterclockwise circularly polarized light while it transmits otherlight. Examples of such a brightness-enhancement film include “PCF 350(trade name)” manufactured by Nitto Denko Corporation, “Transmax (tradename)” manufactured by Merck and Co., Inc., and the like.

An optical member including a laminate of at least two theabove-mentioned optical layers can be formed, for example, by a methodof laminating layers separately in a certain order in the process formanufacturing a liquid crystal display or the like. However, efficiencyin manufacturing a liquid crystal display or the like can be improved byusing an optical member that has been laminated previously because ofits excellent stability in quality, assembling operability, and thelike. Any appropriate adhesion means such as a pressure-sensitiveadhesive layer can be used for lamination as in the above.

Moreover, it is preferable that the optical film, the laminatedpolarizing plate, or the like according to the present invention furtherhas a pressure-sensitive adhesive layer or an adhesive layer so as toallow easier lamination onto the other members such as a liquid crystalcell. They can be arranged on one surface or both surfaces of theoptical film, the laminated polarizing plate, or the like. The materialfor the pressure-sensitive adhesive layer is not particularly limitedbut can be a conventionally known material such as acrylic polymers. Inparticular, the pressure-sensitive adhesive layer having a low moistureabsorption coefficient and an excellent thermal resistance is preferablefrom the aspects of prevention of foaming or peeling caused by moistureabsorption, prevention of degradation in the optical characteristics andwarping of a liquid crystal cell caused by difference in thermalexpansion coefficients, a capability of forming a liquid crystal displaywith high quality and excellent durability, and the like. It also may bepossible to incorporate fine particles so as to form thepressure-sensitive adhesive layer showing light diffusion property. Forthe purpose of forming the pressure-sensitive adhesive layer on thesurface of the optical film, the laminated polarizing plate, or thelike, a solution or melt of a sticking material can be applied directlyon a predetermined surface of the optical film, the laminated polarizingplate, or the like by a development method such as flow-expansion andcoating. Alternatively, a pressure-sensitive adhesive layer can beformed on a separator, which will be described below, in the same mannerand transferred to a predetermined surface of the optical film, thelaminated polarizing plate, or the like.

In the case where a surface of a pressure-sensitive adhesive layer or anadhesive layer provided on the optical film, the laminated polarizingplate, or the like is exposed, it is preferable to cover the surfacewith a separator tentatively so as to prevent contamination until thepressure-sensitive adhesive layer or the adhesive layer is put to use.The separator can be made of a suitable film, e.g., the above-mentionedtransparent protective film, coated with a peeling agent if required.The peeling agent may be selected, for example, from a silicone-basedagent, a long-chain alkyl-based agent, a fluorine-based agent, an agentcontaining molybdenum sulfide, and the like.

The respective layers such as the polarizer, the transparent protectivelayer, the pressure-sensitive adhesive layer, or the adhesive layer forcomposing the optical film or the laminated polarizing plate accordingto the present invention may be subjected to a suitable treatment suchas a treatment with an UV absorber, e.g., salicylate ester compounds,benzophenone compounds, benzotriazole compounds, cyanoacrylatecompounds, or nickel complex salt-based compounds, thus providing an UVabsorbing capability.

The optical film and the laminated polarizing plate according to thepresent invention can be used preferably for forming various devicessuch as liquid crystal displays. For example, a polarizing plate can bearranged on at least one surface of a liquid crystal cell so as to beapplied to, for example, a reflection-type, semi-transmission-type, ortransmission and reflection type liquid crystal display. A liquidcrystal cell to compose the liquid crystal display can be selectedarbitrarily. For example, it is possible to use liquid crystal cells ofappropriate types such as active matrix driving type represented by athin film transistor type, a simple matrix driving type represented by atwist nematic type and a super twist nematic type.

Examples of the liquid crystal cell include STN (Super Twisted Nematic)cells, TN (Twisted Nematic) cells, IPS (In-Plane Switching) cells, VA(Vertical Aligned) cells, OCB (Optically Aligned Birefringence) cells,HAN (Hybrid Aligned Nematic) cells, ASM (Axially Symmetric AlignedMicrocell) cells, ferroelectric cells, and antiferroelectric cells. Thecells may be subjected to an alignment-division systematically orrandomly. The birefringent layer according to the present invention isexcellent particularly in optical compensation of VA (Vertical Aligned)cells.

FIG. 4 shows an example of a liquid crystal display including an opticalfilm according to the present invention, a liquid crystal cell, apolarizer, and a protective film. As shown in FIG. 4, an optical film(1) of the present invention is arranged between a liquid crystal cell(21) and a polarizer (2). On the surface of the polarizer (2) other thanthe surface in contact with the optical film, a protective film (3) isarranged.

Since the optical film according to the present invention are excellentparticularly in optical compensation of a VA (Vertical Aligned) cell,they are most suitably used for viewing-angle compensating films for VAmode liquid crystal displays.

In general, a typical liquid crystal cell is composed of opposing liquidcrystal cell substrates and a liquid crystal injected into a spacebetween the substrates. The liquid crystal cell substrates can be madeof glass, plastics, or the like without any particular limitations.Materials for the plastic substrates can be selected from conventionallyknown materials without any particular limitations.

When polarizing plates or optical members are arranged on both sides ofa liquid crystal cell, the polarizing plates or the optical members onthe surfaces can be the same or different type. Moreover, for forming aliquid crystal display, one or more layers of appropriate members suchas a prism array sheet, a lens array sheet, an optical diffuser, and abacklight can be arranged at proper positions.

The optical film (birefringence film) and the laminated polarizing plateaccording to the present invention can be used not only in theabove-described liquid crystal displays but also in, for example,self-light-emitting displays such as organic electroluminescence (EL)displays, plasma displays (PD) and field emission displays (FED). Whenthe optical film or the laminated polarizing plate of the presentinvention is used in a self-light-emitting flat display, the opticalfilm or the laminated polarizing plate can be used as an antireflectionfilter because circularly polarized light can be obtained by setting thein-plane retardation value And of the birefringent layer included in theoptical film to λ/4, for example.

The following is a specific description of an electroluminescence (EL)display including a laminated polarizing plate according to the presentinvention. The EL display of the present invention is a display havingan optical film or a laminated polarizing plate according to the presentinvention, and can be either an organic EL display or an inorganic ELdisplay.

In recent EL displays, for preventing reflection from an electrode in ablack state, use of an optical film such as a polarizer and a polarizingplate as well as a λ/4 plate is proposed. The laminated polarizing plateand the optical film according to the present invention are especiallyuseful when linearly polarized light, circularly polarized light, orelliptically polarized light is emitted from an EL layer. The polarizingplate with optical compensation function according to the presentinvention is especially useful even when an oblique light beam ispartially polarized even in the case where natural light is emitted in afront direction.

First, a typical organic EL display will be explained below. In general,such an organic EL display has a ruminant (organic EL luminant) that isprepared by laminating a transparent electrode, an organic luminantlayer, and a metal electrode in this order on a transparent substrate.Here, the organic luminant layer is a laminate of various organic thinfilms. Examples thereof include various combinations such as a laminateof a hole injection layer made of a triphenylamine derivative or thelike and a ruminant layer made of a phosphorous organic solid such asanthracene; a laminate of the ruminant layer and an electron injectionlayer made of a perylene derivative or the like; and a laminate of thehole injection layer, the luminant layer, and the electron injectionlayer.

In general, the organic EL display emits light on the followingprinciple: a voltage is applied to the anode and the cathode so as toinject holes and electrons into the organic ruminant layer, energygenerated by the re-bonding of these holes and electrons excites thephosphor, and the excited phosphor emits light when it returns to thebasis state. The mechanism of the re-bonding of these holes andelectrons during the process is similar to that of an ordinary diode.This implies that current and the light emitting intensity exhibit aconsiderable nonlinearity accompanied with a rectification with respectto the applied voltage.

It is preferred for the organic EL display that at least one of theelectrodes is transparent so as to obtain luminescence at the organicruminant layer. In general, a transparent electrode of a transparentconductive material such as indium tin oxide (ITO) is used for theanode. Use of substances having small work function for the cathode isimportant for facilitating the electron injection and thereby raisingluminous efficiency, and in general, metal electrodes such as Mg—Ag andAl—Li can be used.

In an organic EL display configured as described above, it is preferablethat the organic ruminant layer is made of a film that is extremely thinsuch as about 10 nm, so that the organic luminant layer can transmitsubstantially whole light as the transparent electrode does. As aresult, when the layer does not illuminate, a light beam entering fromthe surface of the transparent substrate passes through the transparentelectrode and the organic ruminant layer and is reflected at the metalelectrode so that it comes out again to the surface of the transparentsubstrate. Thereby, the display surface of the organic EL display lookslike a mirror when viewed from exterior.

In an organic EL display according to the present invention including anorganic EL luminant having a transparent electrode on the surface sideof an organic luminant layer and a metal electrode on the back surfaceof the organic luminant layer, for example, it is preferable that anoptical film (a polarizing plate or the like) according to the presentinvention is arranged on the surface of the transparent electrode, andfurthermore, a λ/4 plate is arranged between the polarizing plate and anEL element. As described above, an organic EL display obtained byarranging an optical film according to the present invention cansuppress external reflection and improve the visibility. It is furtherpreferable that a retardation plate is arranged between the transparentelectrode and the optical film.

The retardation plate and the optical film (the polarizing plate or thelike) polarize, for example, light which enters from outside and isreflected by the metal electrode, and thus the polarization has aneffect that the mirror of the metal electrode cannot be viewed fromexterior. Particularly, the mirror of the metal electrode can be blockedcompletely by forming the retardation plate with a quarter wavelengthplate and adjusting an angle formed by the polarization directions ofthe retardation plate and the polarizing plate to be π/4. That is, thepolarizing plate transmits only the linearly polarized light componentamong the external light entering the organic EL display. In general,the linearly polarized light is changed into elliptically polarizedlight by the retardation plate. When the retardation plate is a quarterwavelength plate and when the angle is π/4, the light is changed intocircularly polarized light.

This circularly polarized light passes through, for example, thetransparent substrate, the transparent electrode, and the organic thinfilm. After being reflected by the metal electrode, the light passesagain through the organic thin film, the transparent electrode, and thetransparent substrate, and turns into linearly polarized light at theretardation plate. Moreover, since the linearly polarized light crossesthe polarization direction of the polarizing plate at a right angle, itcannot pass through the polarizing plate. Consequently, as describedabove, the mirror of the metal electrode can be blocked completely.

EXAMPLES

The following is a more specific description of the present invention byway of examples and comparative examples, though the present inventionis by no means limited to the examples below. The characteristics ofoptical films are evaluated in the following manner.

The retardation was measured using a retardation meter (manufactured byOji Scientific Instruments, trade name: KOBRA 21ADH).

The refractive index at 590 nm was measured using a retardation meter(manufactured by Oji Scientific Instruments, trade name: KOBRA 21ADH).

The film thickness was measured using a digital micrometer K-351Cmanufactured by Anritsu Corporation.

The viscosity of solutions was measured using a rheometer (manufacturedby Thermo Haake GmbH). The measurement temperature was 25° C.

Example 1

Polyimide having a weight-average molecular weight (Mw) of 110,000 wasfirst synthesized from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane(6FDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMBTFMB) andthen dissolved in cyclohexanone to prepare a 15 wt % solution of thispolyimide. The thus-obtained polyimide solution was applied onto a 75 μmthick triacetylcellulose (TAC) film (transparent polymer film) preparedby stretching a TAC film to 1.3 times its original length at 175° C. byfixed-end transverse stretching. Thereafter, the film having a layer ofthe polyimide solution was heat-treated at 100° C. for 10 minutes, thusforming a completely transparent and smooth birefringent layer (a)(polyimide film) having a thickness of 6 μm on the TAC film. In thismanner, an optical film was obtained. The birefringent layer (a) of thisoptical film exhibited optical characteristics satisfying nx>ny>nz.

Example 2

Polyaryletherketone A (trade name) (manufactured by Nippon Shokubai Co.,Ltd.) represented by the above formula (18) and having a molecularweight of 200,000 was dissolved in methyl isobutyl ketone to prepare a20 wt % solution of the Polyaryletherketone A. The thus-obtainedpolyaryletherketone solution was applied onto a 75 μm thicktriacetylcellulose (TAC) film (transparent polymer film) prepared bystretching a TAC film to 1.3 times its original length at 175° C. byfixed-end transverse stretching. Thereafter, the film having a layer ofthe polyaryletherketone solution was heat-treated at 100° C. for 10minutes, thus forming a completely transparent and smooth birefringentlayer (a) having a thickness of 10 μm on the TAC film. In this manner,an optical film was obtained. The birefringent layer (a) of this opticalfilm exhibited optical characteristics satisfying nx>ny>nz.

Example 3

The same polyimide solution as in Example 1 was applied onto a TAC film(transparent polymer film). Thereafter, the film having a layer of thepolyimide solution was heat-treated at 100° C. for 10 minutes, thusforming a completely transparent and smooth birefringent layer (a)having a thickness of 4.2 μm on the TAC film. The thus-obtained laminateof the birefringent layer (a) and the TAC film was stretched 10% at 150°C. by uniaxial-longitudinal stretching. In this manner, an optical filmwas obtained. The optical film has a thickness of 4 μm and thebirefringent layer (a) of this optical film exhibited opticalcharacteristics satisfying nx>ny>nz.

Example 4

Polyimide having a Mw of 30,000 was first synthesized from4,4′-bis(3,4-dicarboxyphenyl)-2,2-diphenylpropane dianhydride and2,2′-dichloro-4,4′-diaminobiphenyl and then dissolved in cyclopentanoneto prepare a 20 wt % solution of this polyimide. The thus-obtainedpolyimide solution was applied onto a TAC film (transparent polymerfilm). Thereafter, the film having a layer of the polyimide solution washeat-treated at 130° C. for 5 minutes and then stretched 10% at 150° C.,thus forming a transparent and smooth birefringent layer (a) having athickness of 5 μm on the TAC film. In this manner, an optical film wasobtained. The birefringent layer (a) of this optical film exhibitedoptical characteristics satisfying nx>ny>nz.

Example 5

Polyimide having a Mw of 100,000 was first synthesized from2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane and2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl and then dissolved incyclohexanone to prepare a 15 wt % solution of this polyimide. Thethus-obtained polyimide solution was applied onto a TAC film(transparent polymer film). Thereafter, the film having a layer of thepolyimide solution was heat-treated at 150° C. for 5 minutes and thenstretched 10% at 150° C., thus obtaining a completely transparent andsmooth birefringent layer (a) having a thickness of 6 μm on the TACfilm. The laminate of the birefringent layer (a) and the TAC film was anoptical film having the birefringent layer that exhibits opticalcharacteristics satisfying nx>ny>nz.

Example 6

First, a solution having a solid content of 15 wt % was prepared bydissolving in methylene chloride 75 weight parts of an alternatingcopolymer of isobutene and N-methyl maleimide (a content of N-methylmaleimide was 50 mol %) and 25 weight parts of acrylonitrile-styrenecopolymer in which a content of acrylonitrile was 28 wt %. This solutionwas flow-expanded on a polyethylene terephthalate (PET) film arranged ona glass sheet, and left at room temperature for 60 minutes so that thesolution hardened to form a film. The film was then peeled off from thePET film and heat-treated at 100° C. for 10 minutes. Subsequently, thefilm was further heat-treated at 140° C. for 10 minutes and at 160° C.for 30 minutes, thus obtaining a transparent polymer film. This film hadan in-plane retardation (Δnd) of 4 nm, Rth of 4 nm, and a birefringence(Δn(b)) of 0.0001.

The same polyimide solution as in Example 1 was applied onto thethus-obtained transparent polymer film. Thereafter, the film having alayer of the polyimide solution was heat-treated at 100° C. for 5minutes, thus forming a completely transparent and smooth birefringentlayer (a) having a thickness of 6.2 μm on the transparent polymer film.The thus-obtained laminate of the birefringent layer (a) and thetransparent polymer film was stretched 10% at 130° C. byuniaxial-longitudinal stretching. In this manner, an optical film wasobtained. The birefringent layer (a) of this optical film has athickness of 6 μm and exhibited optical characteristics satisfyingΔn(a)=about 0.035 and nx>ny>nz.

Example 7

The same polyimide as in Example 1 was dissolved in methyl isobutylketone to prepare a 25 wt % solution of this polyimide. The polyimidesolution was applied onto a TAC film (transparent polymer film).Thereafter, the film having a layer of the polyimide solution washeat-treated at 160° C. for 5 minutes, thus forming a completelytransparent and smooth birefringent layer (a) having a thickness of 6 μmon the TAC film. In this manner, an optical film was obtained. Thebirefringent layer (a) of this optical film exhibited opticalcharacteristics satisfying nx>ny>nz.

Example 8

The same polyimide solution as in Example 1 was applied onto a TAC film(transparent polymer film). Thereafter, the film having a layer of thepolyimide solution was heat-treated at 100° C. for 10 minutes, thusforming a completely transparent and smooth birefringent layer (a)having a thickness of 6 μm on the TAC film. In this manner, an opticalfilm was obtained. The birefringent layer (a) of this optical filmexhibited optical characteristics satisfying nx≈ny>nz.

Comparative Example 1

Polyimide having a weight-average molecular weight (Mw) of 8,000 wasfirst synthesized from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane(6FDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMBTFMB) andthen dissolved in cyclohexanone to prepare a 15 wt % solution of thispolyimide. The thus-obtained polyimide solution was applied onto an 80μm thick triacetylcellulose (TAC) film (transparent polymer film)prepared by stretching a TAC film to 1.2 times its original length at150° C. by longitudinal stretching. Thereafter, the film having a layerof the polyimide solution was heat-treated at 150° C. for 10 minutes,thus forming a completely transparent and smooth birefringent layer (a)(polyimide film) having a thickness of 6 μm on the TAC film. In thismanner, an optical film was obtained. The thus-obtained optical filmhaving the birefringent layer (a) is an optical film having abirefringent layer (a) satisfying nx>ny>nz.

Comparative Example 2

A norbornene-based resin film (manufactured by JSR Corporation, tradename: ARTON film) was stretched to 1.3 times its original length at 150°C. by fixed-end transverse stretching, thus obtaining an 80 μm thickoptical film. This film exhibited optical characteristics satisfyingnx>ny>nz. The polymer forming the norbornene-based resin film had aweight-average molecular weight of 60,000.

Comparative Example 3

A 75 μm thick PET film was prepared by stretching a PET film to 1.3times its original length at 175° C. by fixed-end transverse stretching,and the same polyimide solution as in Example 1 was applied onto thisPET film. Thereafter, the film having a layer of the polyimide solutionwas dried at 150° C. for 5 minutes, thus forming a completelytransparent and smooth birefringent layer (a) having a thickness of 6 μmon the PET film. In this manner, an optical film was obtained. Thebirefringent layer (a) of this optical film exhibited opticalcharacteristics satisfying nx>ny>nz.

Comparative Example 4

Polyetherketone (Mw: 500,000) represented by the above formula (18) wasdissolved in cyclopentanone to prepare a 25 wt % solution of thepolyetherketone. On the other hand, an 80 μm thick triacetylcellulose(TAC) film was stretched to 1.3 times its original length at 175° C. byfixed-end transverse stretching, thus preparing a 75 μm thick TAC film.The above-described polyetherketone solution was applied onto thestretched TAC film. Thereafter, the stretched TAC film having a layer ofthe polyetherketone solution was heat-treated at 100° C. for 10 minutes,thus forming a completely transparent and smooth birefringent layer (a)having a thickness of 75 μm on the stretched TAC film. In this manner,an optical film was obtained. The birefringent layer (a) of this opticalfilm exhibited optical characteristics satisfying nx>ny>nz. Owing to alarge molecular weight of the polyetherketone, it was difficult to applythe polyetherketone solution uniformly onto the stretched TAC film. As aresult, the retardation varied depending on a portion of thebirefringent layer (a). In addition, the polyetherketone was notdissolved in cyclopentanone completely, resulting in a poor appearance.

(Evaluation of Optical Films)

With regard to the optical films obtained in Examples 1 to 8 andComparative Examples 1 to 4, the values of Δn(a), Δn(b), (nx−ny)×d,(nx−nz)×d, and (nx−nz)/(nx−ny) were calculated from the values of nx,ny, and nz. The results are shown in Table 1.

Also, the viscosity of the polyimide solutions applied in Examples 1 to8 and Comparative Examples 1 and 2 to 4 and the thickness of therespective birefringent layers (a) in the respective examples andcomparative examples are shown in Table 1.

The optical films obtained in Examples 1 to 8 and Comparative Examples 1to 4 were stored in a dryer at 100° C. for 100 hours, and the long-termstorage stability of the respective optical films was evaluated. Theresults of the evaluation are shown in Table 1, where “Bad” means thatcracks occurred, while “Good” means that no cracks occurred.

Furthermore, the results of the evaluation on the coating accuracy ofthe optical films obtained in Examples 1 to 8 and Comparative Examples 1to 4 also are shown in Table 1. In Table 1, “Good” means that thesolution could be applied uniformly, thus causing no variation in theretardation, while “Bad” means that it was difficult to apply thesolution uniformly so that the retardation varied depending on a portionof the birefringent layer (a).

TABLE 1 Viscosity Thickness of of polymer (nx − nz)/ birefringentLong-term Mw of solution (nx − ny) · (nx − nz) · (nx − ny) layer (a)storage Coating Δn(b) polymer [Pa · s] Δn(a) d [nm] d [nm] [nm] [μm]stability accuracy iridescence Ex. 1 0.0006 110,000 1 0.045 135 270 2.06 Good Good none Ex. 2 0.0006 200,000 8 0.018 50 180 3.6 10 Good Goodnone Ex. 3 0.0006 110,000 1 0.038 100 150 1.5 4 Good Good none Ex. 40.0006 30,000 1.5 0.025 50 125 2.5 5 Good Good none Ex. 5 0.0006 100,0001 0.039 100 235 2.4 6 Good Good none Ex. 6 0.0001 110,000 1 0.035 80 2102.6 6 Good Good none Ex. 7 0.0006 110,000 12 0.038 70 230 3.3 6 GoodGood none Ex. 8 0.0006 110,000 1 0.037 0.2 220 1100.0 6 Good Good noneCom. 0.0006 8,000 0.5 0.030 60 180 3.0 6 Bad Good none Ex. 1 Com. 0.0006— — 0.002 91 182 2.0 80 Good — none Ex. 2 Com. 0.08 110,000 8 0.042 50250 5.0 6 Good Good observed Ex. 3 Com. 0.0006 500,000 15 0.020 10 20020 10 Good Bad none Ex. 4

As becomes clear from Table 1, the optical films of the respectiveexamples satisfied the above formulae (1) to (3) and the molecularweights of the polymers forming the respective birefringent layers (a)were in the range between 10,000 and 400,000 inclusive, thereby allowingvariations in retardation of the optical films to be suppressed.

Moreover, the viscosities of the polymer solutions used for forming thebirefringent layers (a) in the respective examples were not too high,and the optical films obtained were excellent in long-term storagestability.

(Evaluation of Liquid Crystal Displays Including Optical Films)

To each of the optical films obtained in Examples 1 to 7 and ComparativeExamples 1 to 5, a polarizing plate (manufactured by Nitto DenkoCorporation, trade name: HEG1425DU) was attached with apressure-sensitive acrylic adhesive. Thus, twelve types of laminatedpolarizing plates were obtained. Each of these laminated polarizingplates was attached on a backlight side of a liquid crystal cell with anacrylic adhesive so that the optical film faced the liquid crystal cell.In this manner, liquid crystal displays were produced.

The display characteristics of these liquid crystal displays wereexamined. The presence or absence of iridescence is shown in Table 1above. As shown in Table 1, iridescence was not observed in the opticalfilms according to the respective examples.

INDUSTRIAL APPLICABILITY

As specifically described above, an optical film according to thepresent invention can prevent the occurrence of iridescence, suppressesvariation in retardation and a poor appearance, and has excellentlong-term storage stability. As a result, by using the optical film ofthe present invention in, for example, a liquid crystal display, it ispossible to improve a display quality over a long period.

1. An optical film comprising: a transparent polymer film (b); and abiaxially birefringent layer (a) formed of a polymer and laminated abovethe transparent polymer film (b), wherein the birefringent layer (a) andthe transparent polymer film (b) satisfy a formula (1) below, thebirefringent layer (a) satisfies formulae (2) and (3) below, and thepolymer forming the birefringent layer (a) has a weight-averagemolecular weight in a range between 10,000 and 400,000 inclusive,Δn(a)>Δn(b)×10  (1)1<(nx−nz)/(nx−ny)<100  (2)0.0005≦Δn(a)≦0.5  (3) where Δn(a) is a birefringence of the birefringentlayer (a) and is represented by [(nx+ny)/2]−nz, Δn(b) is a birefringenceof the transparent polymer film (b) and is represented by[(nx′+ny′)/2]−nz′, nx, ny, and nz represent refractive indices in anX-axis direction, a Y-axis direction, and a Z-axis direction in thebirefringent layer (a), respectively, with the X-axis direction being anaxial direction exhibiting a maximum refractive index within a plane ofthe birefringent layer (a), the Y-axis direction being an axialdirection perpendicular to the X-axis within the plane, and the Z-axisdirection being a thickness direction perpendicular to the X-axis andthe Y-axis, and nx′, ny′, and nz′ represent refractive indices in anX-axis direction, a Y-axis direction, and a Z-axis direction in thetransparent polymer film (b), respectively, with the X-axis directionbeing an axial direction exhibiting a maximum refractive index within aplane of the transparent polymer film (b), the Y-axis direction being anaxial direction perpendicular to the X-axis within the plane, the Z-axisdirection being a thickness direction perpendicular to the X-axis andthe Y-axis.
 2. The optical film according to claim 1, wherein thebirefringent layer (a) is laminated directly on the transparent polymerfilm (b).
 3. The optical film according to claim 1, wherein the polymerforming the birefringent layer (a) is a non-liquid crystalline polymer.4. The optical film according to claim 3, wherein the non-liquidcrystalline polymer is at least one polymer selected from the groupconsisting of polyamide, polyimide, polyester, polyetherketone,polyaryletherketone, polyamide imide, and polyesterimide.
 5. The opticalfilm according to claim 1, wherein the transparent polymer film (b) is aprotective film for a polarizer.
 6. A laminated polarizing platecomprising an optical film, wherein the optical film is the optical filmaccording to claim
 1. 7. A liquid crystal panel comprising a liquidcrystal cell and an optical member, the optical member being arranged onat least one surface of the liquid crystal cell, wherein the opticalmember is the laminated polarizing plate according to claim
 6. 8. Aliquid crystal display comprising a liquid crystal panel, wherein theliquid crystal panel is the liquid crystal panel according to claim 7.9. A self-light-emitting display comprising the laminated polarizingplate according to claim
 6. 10. A display comprising the laminatedpolarizing plate according to claim
 6. 11. The laminated polarizingplate according to claim 6, wherein the transparent polymer film (b) isa protective film for a polarizer.
 12. A liquid crystal panel comprisinga liquid crystal cell and an optical member, the optical member beingarranged on at least one surface of the liquid crystal cell, wherein theoptical member is the optical film according to claim
 1. 13. A liquidcrystal display comprising a liquid crystal panel, wherein the liquidcrystal panel is the liquid crystal panel according to claim
 12. 14. Aself-light-emitting display comprising the optical film according toclaim
 1. 15. A display comprising the optical film according to claim 1.16. An optical element comprising the optical film of claim 1 and abirefringent film.
 17. The optical film according to claim 1, wherein(nx−nz)/(nx−ny)≦80.
 18. The optical film according to claim 1, wherein(nx−nz)/(nx−ny)≦50.
 19. The optical film according to claim 1, wherein(nx−nz)/(nx−ny)≦30.